Pixellated Arrays

Linear or 2D

  • Minimal pixel separation
  • Reduced optical crosstalk
  • Excellent pixel to pixel uniformity
  • Array to array (batch to batch) uniformity

Offering pixellated array assemblies that include crystals such as LYSO, CdW04, CsI(Tl) and BGO or other materials such as scintillating plastic. Saint-Gobain can also produce crystal arrays made from other materials not grown by Saint-Gobain.  We offer a variety of design options and reflector materials to optimize array performance for your application. Our manufacturing process ensures high light output as well as excellent pixel-to-pixel uniformity with minimal crosstalk.

The optimal material choice involves a combination of factors including the application, the photo readout device to be used, etc. For example, if the application calls for rapid detection of radiation pulses, then the decay time and afterglow drive the choice (BGO, LYSO, CdWO4). If high efficiency and low cost are paramount, then NaI(Tl) with a PMT readout or CsI(Tl) with a photodiode readout are the first detectors to consider. The physical properties of each material must be considered as well. For example, the natural cleavage plane in CdWO4 restricts the minimum size of 2D pixels.

The type of material depends on the application, and the information being sought.
 

Applications Well-suited for Scintillation Array Materials
Material CsI(Tl) CdWO4 BGO LYSO
Gamma Cameras X      
CT Imaging X X    
Positron Emission Tomography X   X X
Line Scanning X X    
Industrial Imaging X X X  
Flash Radiography     X X
Digital Radiography X   X  
Design

There are many design options in terms of pixel size, the number of pixels, pixel separator materials (reflective and/or radiation barrier) and crystal surface finishes.

Linear Array Assemblies (single row of pixels) – Linear array assemblies are typically used with pixelated photodiodes and used quite often in security type applications, most common are baggage scanners, cargo inspection, and other non-destructive testing.   When used in a medical application, crystals such as LYSO are used in Bone Mineral Densitometry machines. 

2D Array Assemblies (pixels arranged in an X-Y matrix) – Two-dimensional arrays are typically used with pixelated photodiodes, position-sensitive photomultiplier tubes (PSPMT) and silicon photomultipliers (SiPM) and most common use is in medical imaging scanners (PET, SPECT, CT), and non-destructive testing and inspection. 

Array Design Parameters

There are choices of scintillator materials and separator/reflectors to optimize performance for a specific application. The listing of parameters addresses the elements that must be considered in the design of a linear or 2D array.

Design Parameters

  • Material: Type of scintillation crystal or material desired.
  • Pixel or Element Size: The “X” and “Y” dimensions of each scintillator pixel.
  • Separator Type and Thickness: The type of reflector between the crystal pixels and its overall thickness, “Gap X(A)” or “Gap Y(B)”.  Note: this may be a composite or laminate of white reflector and metal materials. The geometry of the pixel, the thickness of the reflector, the scintillator material used and other factors influence the reflectivity obtained in each array design. Array reflector materials are listed in the order of decreasing reflectivity.
  • Pitch: This is the distance between the center of one element to the center of an adjacent element, “X” + “Gap X(A)” or “Y” + “Gap Y(B)” Note: In 2D arrays with rectangular pixels, the pitches in the “X” and “Y” directions will be different.
  • Radiation Thickness: This is the “Z” dimension and specifies the thickness of the array in the direction of the incoming radiation.
  • Back Reflector Thickness: Usually a white reflector is applied to the radiation entrance side of the array to reflect the light back into the pixel so it can be directed to the light sensor.
  • Material adjacent to the end pixels or elements: The end crystals may need a special reflector thickness or other treatment, e.g., to keep a constant pitch from array to array if they will be joined together in use.
1D Array 2D Array
1D Linear Array
Two Dimensional Array
Pixel Sizes Available

The table below shows the materials and the associated pixel sizes that are regularly produced today. The pixel sizes are controlled primarily by the mechanical properties of the crystals, e.g. hardness, cleavage, ease of machining. For example, CdWO4 has a cleavage plane in one crystallographic direction. For that reason, it is not possible, with current techniques, to achieve 0.3 x 0.3 mm2 pixels because of fractures along the cleavage planes that occur during cutting and grinding in manufacture. However, 0.3 x 1.0 mm2 pixels can be produced.

 

Minimum Discrete Pixel Sizes Available in Crystal Scintillators
Material Minimum Pixel Sizes* Comments
Linear (mm) 2D (mm)
CsI(Tl) 0.3 0.5  
CdWO4 0.3 1.0 Cleavage Plane
BGO 0.3 0.3  
LYSO 0.8 0.8 Min. Untested
  *Guidelines, not hard numbers  

 

Separator/Reflector

The geometry of the pixel, the thickness of the reflector, the scintillator material used and other factors influence the reflectivity obtained in each array design. Array reflector materials are listed in the order of decreasing reflectivity. The reflectivity numbers are presented as a guide only. The first two separator materials listed (white powder and Teflon sheet) are not practical for most of the small pixel arrays discussed here – they cannot provide the bonding properties required. However, they are useful in some encapsulated units. Once mixed with epoxy, the white powder provides the diffuse reflectivity required to channel the scintillation light to the exit surface and the adhesive properties for a mechanically stable array.

Metal or metalized separators prevent optical crosstalk between the pixels while maintaining minimum gap “G” thicknesses. However, the metal surfaces, even polished, do not provide the best reflection of the scintillation light to the exit surface. This is where composites are useful. They combine the reflective properties of the white materials with the “zero” optical crosstalk of solid metals or films. Metal separators can serve another function: to absorb the radiation that is incident on the separator area before it strikes the light sensor and produces noise. Nuclear dense materials like Lead, Tungsten, and Tantalum are used. Also available are white epoxies where the reflector particle fillers are more nuclear dense than TiO2 or Al2O3. However, in practice, their effectiveness is limited to low energies, up to 60keV.

 

Separator Types and Thicknesses in Order of Decreasing Reflectivity
Material Thickness Range Approximate Relative Reflectivity *
White Powder (e.g. TiO2, MgO) ** 0.25 mm and up 100%
Teflon Sheet ** 0.15 mm - 0.50 mm 98%
White Reflector Paint 0.04 mm - 0.10 mm 96%
White Plastic 0.05 mm and up 95%
White Epoxy 0.10 mm - 0.75 mm 94%
Composites *** 0.10 mm and up 94%
Aluminum/Epoxy 0.05 mm - 0-.1 mm 75%
Metals (Pb, Ta) / Epoxy 0.05 mm and up 65%

 

* These are guidelines only and are based on optimum, not minimum, thickness.
   Values will vary with pixel geometry, surface finish and other specific design parameters.
** These are used as reflector materials in large scintillation crystal packaging.
*** Composite separators are clear epoxy-paint-clear epoxy, white epoxy- metal-white epoxy.

 

Array Scintillator Properties
Material CsI(Tl) CdWO4 BGO LYSO
Afterglow 0.5-5% @6ms 0.1% @3ms 0.005% @3ms <0.1% @6ms
Primary Decay Time (ns) 1000 14000 300 36
Wavelength of Maximum Emission [nm] 550 475 480 420
Relative Rad Hardness Medium High High High
Relative Light Output [photons/keV] 54 13 9 32
Hygroscopic slightly no no no
Solubility in H2O, g/100g@25oC 85.5 0.5
Density, g/cm3 4.51 8.0 7.13 7.10
Understanding Array Model Numbers
  1D Array 2D Array
  Example Model Numbers 82.58X4.2A30/16/5.2CsI(Tl) 82.58X4.2A30/16x4/5.2x4CsI(Tl)
1 Active area length 82.58 82.58
2 Active area height 4.175 4.175
3 X-ray crystal depth (Z) 30 30
4 Number of pixels
If the array is 2D, this is in the format [Pixels X] x [Pixels Y]
16 16x4
5 Pitch [X + Gap X(A)]
If the array is 2D AND the pitch is different in X and Y, this is in the format [X+GapX(A)]x[Y+GapY(B)].
5.2 5.2x4
6 Scintillator CsI(Tl) CsI(Tl)
   
1D Linear Array
Two Dimensional Array
Low Afterglow Linear CsI(Tl) Segmented Arrays

Recent advances in CsI(Tl) array manufacturing have resulted in afterglow reduction, improved light output, and afterglow uniformity. Single energy, dual-energy, fixed frame, rotating gantry, CsI(Tl) based arrays can be used in almost any high-quality X-Ray imaging application in a multitude of industries (Security Baggage Scanning, Cargo Scanning, Medical, Non-Destructive Industrial Inspection).

Array Performance (Typical)
Light output uniformity ±10% within an array (requires matching photodiode)
Light output array to array ±10%
Afterglow 5000ppm @100ms (initial test)
≤2500ppm (after burn-in)
Afterglow uniformity ±10% within an array
Array Design Capabilities
Number of channels (typical) 8-64
Minimum pitch 0.5mm
X-Ray thickness 50mm Max

See the product performance sheet for test parameters.

 

Brochures
Array-Brochure.pdf

Scintillation Array - Linear and 2D

PDF | 1.46 MB
Array-Brochure
Technical papers
Low-Afterglow-Linear-CsITl-Segmented-Arrays.pdf

Low Afterglow Linear CsI(Tl) Segmented Arrays

PDF | 662.69 KB
Low-Afterglow-Linear-CsITl-Segmented-Arrays